The present invention relates to a cooling device of a turbocharger of an engine for a vehicle, and in particular relates to the one in which a first turbocharger and a second turbocharger are provided at a one-side wall of the engine, the second turbocharger being located above the level of the first turbocharger.
Conventionally, a turbo charging device to supercharge intake air by using the kinetic energy of exhaust gas exhausted from an engine is known. Further, a sequential type of turbo charging device, in which there are provided a small-sized turbocharger to supercharge the intake air mainly at a low engine-speed and low load and a large-sized turbocharger to supercharge the intake air mainly at a high engine-speed and high engine-load, and supercharging characteristics of the turbochargers are selectable in accordance with an engine's operation state, is known.
In the turbo charging device disclosed in US Patent Application Publication No. 2003/150408, the large-volume first turbocharger and the small-volume second turbocharger are provided at the one-side wall of the engine, the second turbocharger being located above the level of the first turbocharger, the compressor of the second turbocharger being arranged downstream of the compressor of the first turbocharger, and the intercooler is arranged upstream of the coolant's cooler and between the compressor of the first turbocharger and the compressor of the second turbocharger. According to this device, the length of the intake-air pipe between the intercooler and the compressor of the second turbocharger can be shorter, so that the layout space can be improved.
In the turbo charging device disclosed in Japanese Patent Laid-Open Publication No. 2006-70878, the small-sized turbocharger, the large-sized turbocharger, the DPF (Diesel Particulate Filter) connected to the turbine outlet of the large-sized turbocharger are provided at the one-side wall of the engine, the large-sized turbocharger being arranged below the small-sized turbocharger, the DPF being located below the small-sized turbocharger and substantially at the level of the large-sized turbocharger so that the exhaust-gas induction port thereof opens on the side of the turbine of the large-sized turbocharger. According to this device, the DPF and turbochargers can be provided compactly on the one-side wall of the engine, and the exhaust gas with less exhaust-energy loss and higher temperature can be obtained.
The turbine shaft of the turbo charging device is rotatably supported at the oil-lubrication type of the shaft bearing portion in the center housing. Herein, the turbine driven by the exhaust-gas kinetic energy rotates at a high engine-speed exceeding 200,000 rpm, so the shaft bearing portion receives a high thermal load and therefore it is cooled with the cooling water circulating the center housing. An operation of a water pump to force the cooling water to circulate is stopped concurrently with a stop of the engine. Therefore, when the engine stops after the engine operation at a high engine load, the cooling water comes to a boil and the vapor of the cooling water generates in the center housing, so that the temperature of the center housing including the shaft bearing portion increases promptly, which may cause a heat damage of the shaft bearing portion or deterioration of the lubricating oil. Accordingly, the vapor inside the center housing is generally released to a cooling-water storage portion provided on the outside of the engine body, for example, a radiator or the like.
In the turbo charging device disclosed in Japanese Patent Laid-Open Publication No. 2003-239752, there are provided the turbocharger provided at the front side wall of the engine, the first cooling-water passage as a vapor releasing passage to connect the upper tank of the radiator and the turbocharger, the second cooling-water passage to connect the turbocharger and the heater return passage provided on the suction side of the water pump, and so on. Thereby, when the engine stops after the engine operation at the high load or the like, the vapor generating in the center housing flows into the upper tank through the first cooling-water passage and the same amount of cooling water as the cooling water flowing into the upper tank is newly supplied from the second cooling-water passage.
According to the turbo charging device disclosed in the above-described third patent publication, the vapor can be discharged effectively from the inside of the center housing by using the cooling-water passage. However, in the case of providing the two turbochargers at the side wall of the engine as disclosed in the above-described first and second patent publications, the following new problems may occur.
First, it is necessary that the difference in the level between the center housing and the flowing-out position (the connection portion) of the vapor releasing passage is set at a specified height or greater in order to secure the vapor releasing function of the respective turbochargers. However, in case there is a level difference between the connection portions of the respective vapor releasing passages provided at the identical cooling-water storage portion, the vapor flowing out from the connection portion of the vapor releasing passage provided at the lower position flows in from the connection portion of the vapor releasing passage provided at the higher position. As a result, the flowing-out of the vapor from the connection portion provided at the higher position may be obstructed.
Secondly, since the two turbochargers are provided at the engine side wall, in addition to a connection passage for exhaust interconnecting the two turbochargers, a housing, oil supply and return passages, cooling-water supply and return passages and the like are necessary for each of the turbochargers, so that the layout around the engine becomes difficult. Further, in case the exhaust manifold of the engine is positioned on the vehicle rear side, that is—a so-called engine rear exhaust, the length of the cooling-water passages becomes longer compared to a so-called engine front exhaust as disclosed in the third patent publication, and there is a concern that the layout may deteriorate.